A typical air meter for measuring air intake into an internal combustion engine operates on the principle of hot-film anemometry. A heated element is placed within the airflow stream, and maintained at a constant temperature differential above the air temperature. The amount of electrical power required to maintain the heated element at the proper temperature is a direct function of the mass flow rate of the air past the element.
The measurement function of the typical mass air flow meter is performed using a bridge circuit, often referred to as a Wheatstone bridge, and shown as element 10 in FIG. 2. In this circuit, temperature-sensitive resistors are used as the ambient temperature sensor A1 and as the heated sensing element H1. Typical mass air flow measurement in an automotive environment employs a hot wire anemometer, with heated sensor H1 and ambient sensor A1. Both sensors have a high temperature coefficient of resistance. The heated sensor H1 is placed in the air flowstream, and as air flows across the heated sensor H1 heat is removed from the sensor in proportion to mass of air. Resistance of ambient sensor A1 changes with the temperature of the ambient air. The right side of the bridge, consisting of the ambient sensor A1 and calibration resistors (RS and RC), establishes a voltage (Vr) at Node 3 which is based upon ambient temperature. The heated sensing element H1 on the left side of the bridge and the resistor Rp have low resistances relative to the resistors on the right side of the bridge. Power dissipation due to a given voltage is inversely proportional to resistance (power=V2/R), and therefore the low resistance elements H1, Rp on the left side of the bridge dissipate enough electrical power to cause self-heating. The sensing elements are temperature-sensitive resistors and any change in temperature due to self-heating will result in a change in resistance of H1. This affects the voltage divider ratio on the left side of the bridge, and thus the voltage (VL), measured at Node 2. The sensing bridge is balanced when the voltages (VL and VR) are equal. A feedback amplifier 26 operates a FET transistor 30 to adjust electrical potential at Node 1 to maintain a balanced bridge voltage. Consequently, the desired operating temperature of the heated elements is maintained. The bridge voltage, which is the electrical potential at Node 1, is a measure of the heat dissipation at resistor H1, compensated by the ambient sensor A1, and is therefore proportional the mass of air flowing past the sensor.
In typical engine operation, especially with an engine having fewer than six cylinders, the airflow in the intake manifold experiences severe pulsations caused by engine dynamics related to opening and closing of intake valves and associated flow of air into each cylinder. There are areas of operation when airflow reverses, i.e., air flows out of the intake manifold away from the engine. A typical uni-directional air flow sensor is operable to measure magnitude of air flow, but not direction of the airflow. The inability to determine direction of airflow may result in introduction of significant errors in measure of mass air flow into the engine during conditions wherein reverse flow conditions occur.
One potential solution for this problem comprises mounting four sensing resistors on a thin (˜2 micrometer thick) membrane, with a heater in the center of the membrane providing heat using the same bridge voltage as previously described. A constant temperature is maintained in the center of the membrane. Air flowing across the upstream side of the membrane is cooled while the downstream side experiences slight heating. When the sensors are arranged in a bridge circuit, both magnitude and direction of the flow can be measured by comparing the voltage of the two sides of the bridge. However the sensor output signal is extremely small (i.e. range of millivolts) and the corresponding signal-to-noise ratio is not large enough to allow reliable measurement. Furthermore, it is difficult to manufacture and process the sensor, including placement of the sensors in the middle of the membrane. Incorrect placement of sensors may result in a drift of the output signal over time. Most importantly, such a membrane has proven to be fragile, thus reducing reliability of the device, causing customer dissatisfaction and high warranty costs.
Another potential solution implemented includes attempts to mechanically block exposure of a sensing device to reverse flow conditions. This solution reduces error, but there still exists significant flow error.
Another potential solution for the aforementioned problem comprises developing a sophisticated filtering system to monitor signal input from a sensor, and identify reverse flow conditions. A sophisticated filtering system, comprising elements including second-order digital filters and other elements consume substantial amounts of execution time, microprocessor time and computer memory, and is not feasible for implementation in a low-cost microcontroller used primarily in an airflow sensing device.
Therefore, what is needed is a method and apparatus that employs the currently available uni-directional air meter design with additional circuitry and algorithms to effectively measure air flow during forward and reverse flow conditions to provide an accurate measure of net mass airflow.